Frequency mixing, which is also called frequency conversion in terms of engineering, is a process of converting one value of a frequency of a signal into another value. A circuit that implements this frequency mixing function is called a frequency mixing circuit or frequency conversion circuit, and is also called a frequency mixer or frequency converter.
The frequency mixing circuit has been widely applied to communications engineering and wireless technologies. Generally, every wireless communications apparatus has a transmitter and a receiver, where the transmitter is configured to convert user data into a radio frequency signal for data transmission, and the receiver is configured to convert a received radio frequency signal into user data for data receiving.
The frequency mixing circuit plays an important role in transmitting/receiving a radio frequency signal, no matter in a transmitter or a receiver. In the transmitter, the frequency mixing circuit mixes an input signal (a baseband signal or an intermediate frequency signal) with a local oscillation signal to generate a to-be-transmitted radio frequency signal. In the receiver, the frequency mixing circuit mixes a radio frequency signal received by an antenna with a local oscillation signal to generate a baseband signal or an intermediate frequency signal.
The frequency mixing circuit is more widely applied especially in a superheterodyne receiver, for example, converting an amplitude-modulated 535 KHZ-1605 KHZ signal into a 465 KHZ intermediate frequency signal by an AM broadcast receiver, converting a modulated 48.5 M-870 M image signal into a 38 MHZ intermediate frequency image signal by a television receiver, and one intermediate frequency and double intermediate frequency in mobile communications.
A superheterodyne architecture is a main frequency-conversion manner of a current microwave transmitter, as shown in FIG. 1, which mixes information or an intermediate frequency modulation signal IF with a local oscillation signal Lo whose frequency is higher, and boosts a modulation signal carrier to a radio frequency RF.
A frequency of a signal carrier is converted from fIF to fRF, and their relation may be fRF=fLO+fIF or fRF=fLO−fIF. Due to nonlinearity of the frequency mixing circuit, frequency components of an output signal may be expressed as fRF=n*fLO+m*fIF, where n and m are integers.
If fRF=fLO+fIF is taken as an output signal of the frequency mixing circuit, generally, in addition to this signal, a spurious signal fLO is further included at an RF outputting port of the frequency mixing circuit, that is, local oscillation leakage occurs.
To reduce the local oscillation leakage of the frequency mixing circuit, a filter may be used in the frequency mixing circuit. As shown in FIG. 2, by using frequency selectivity of the filter, a radio frequency signal can pass, and a local oscillation frequency is suppressed.
By adopting the foregoing solution to reduce the local oscillation leakage of the frequency mixing circuit, the filter needs to have better frequency selectivity, and the interconnection between the filter and a system is relatively complicated; and furthermore, performance of the filter is greatly affected by a process capability, so that implementability is poor. In addition, because the filter is an element that has frequency selectivity, and a frequency selectivity feature contradicts with wide area coverage of a frequency, this solution cannot satisfy a requirement of a broadband application of the frequency mixing circuit.